no wieght loss and no wieght gain?

My question is, how come i cant gain any wieght and im not lossing any weight either. im 17 im very active in my free time i dont drink, i smoke marijuana but other then that i keep up with my diabetes testing, and recently i just got the pump (which i love it so far) i only wiegh around 120 which is way to low for my age and for some reason i can not gain any wieght, i have been on a high carb diet for almost a year now and nothing has worked, i cant explain it. if anyone knows something i dont please inform me. i kno smokeing increases ur metablosim but i quit for a couple months and still did not gain any wieght and all the doctor can say is stick with ur hight carb diet.

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